Hello, Jen! Welcome to the Disney Parks Moms Panel. I took my daughter on her first Disney Parks vacation when she was around the same age as your child!
I suggest visiting the Royal Hall at
Fantasy Faire to meet princesses first thing in the morning. Cinderella typically meets here along with two other princesses. However, the princess lineup is subject to change. Check the board at the queue entrance or ask a Cast Member which princesses are available the day you are visiting. You can also check the
Disneyland Mobile App to see if Cinderella (and a whole bunch of other characters) is meeting Guests that day.
You might also consider having breakfast or lunch at
Ariel's Grotto in Disney California Adventure Park. You will meet Ariel as you enter the restaurant and then mingle with a few of her princess friends while you dine. While we are on the subject of Ariel,
The Little Mermaid ~ Ariel's Undersea Adventure was a big hit when my daughter was that age (and still is now!).
Then, as you pointed out, visit
Mickey's Toontown a little later in the day when your daughter needs to burn off some energy. She can climb and explore Goofy's Playhouse, Donald's Boat, and Chip 'n Dale Treehouse.
There is so much for little ones to see and do that your daughter is sure to have a magical time!
-Linda M.